Coherent transmission is increasingly being employed in modern fiber-optic networks, as it is a key enabler in greatly increasing both the spectral efficiency (i.e. number of data bits per Hz of optical spectrum) and the tolerance of links to various transmission impairments (e.g. polarization mode dispersion, chromatic dispersion, etc.). Usually, the spectral efficiency is increased by an additional factor of two by spectrally multiplexing orthogonally-polarized optical signals, an approach referred to as “polarization multiplexing” (PolMux). A receiver suitable for use with coherent PolMux signals normally comprises high-performance electronics, high-rejection balanced optical detectors, quality polarization optics, and precision optical-interferometer arrangements, in addition to employing a highly-coherent tunable laser to serve as a local oscillator (LO).
In order to provide the economies of scale necessary for widespread adoption of coherent transmission, most commercial systems employ the basic modulation format and data bit rates recommended by the Optical Internetworking Forum (OIF). In particular the OIF has published an Implementation Agreement (OIF-DPC-RX-01.0) that specifies key performance parameters and specifications to which a commercial Integrated Coherent Receiver (ICR) should be in conformity.
Characterization of an ICR may be undertaken at different points along the supply chain, and the thoroughness of such characterization may vary accordingly. For instance, in its own development or qualification laboratories, an ICR manufacturer will likely characterize all possible parameters and specifications under many different conditions. On the other hand, during the actual fabrication/assembly process in high-volume manufacturing, the parameters will likely be measured under specified conditions (e.g. at a predetermined temperature), and full meteorological specifications provided for those specifications deemed most critical. In many cases, the manufacturer may tightly control key aspects of the fabrication process and thereby have the confidence to guarantee several ICR parameters “by design”. These parameters then will be fully verified only for a small subset of the manufactured ICRs, in order to ensure quality control. Thereafter, the end customer, who intends to integrate the ICRs into its transmission equipment, will normally perform an “incoming inspection” of all or a sample of the delivered ICRs to verify one or more key parameters, in accordance with the same definitions provided in the OIF Implementation Agreement.
The sophistication, and hence cost, of the test-and-measurement instrumentation needed to undertake the above-described characterization varies significantly according to the point along the “supply chain”. The high cost of versatile high-performance test equipment (e.g. high-speed real-time multi-port oscilloscopes) would generally not be critical in an R&D laboratory, since this equipment would also be used for a wide variety of high-performance measurements. On the other hand, there is a need for dedicated test & measurement equipment of lower cost to carry out ICR verification during volume manufacturing, this equipment having features specifically targeted to the measurement of the required parameters.
A key performance parameter that normally must be measured is the Common Mode Rejection Ratio (CMRR). The ICRs are based on optical intradyne coherent detection (where “intradyne” detection refers to a special case of heterodyne detection, for which the local-oscillator optical frequency falls within the spectral bandwidth of the data-carrying signal). Doubly-balanced detection of the “complementary” (i.e. exhibiting a mutual 180-degree phase difference) pair of optical signals leads to nearly complete suppression of contributions to the electrical signal originating from the “direct” optical signal power (|Esig|2) and/or from the “direct” LO light level (|ELO|2), leaving only the data-carrying contribution arising from the mixed signal and LO electric fields in the differential electrical signal. The CMRR quantifies the degree to which these direct terms are suppressed with respect to the differential electrical signal. Typically, the CMRR is dependent upon the frequency of the differential electrical signal. An ICR may exhibit a non-zero CMRR as a result of non-idealities along its optical paths, e.g. non-ideal splitting ratios (e.g. different than 3 dB) or inaccurate path differences within one of the optical interferometers. Non-zero CMRR may also arise from the different responsivities of the two “balanced” detectors (and associated front-end electronics), which often exhibit a dependence on the detected frequency response. As defined herein, the CMRR is a combined measure of both the quality of the optical paths and the detector responsivities, without explicitly isolating the relative contribution of each. In other words, unless otherwise specified, CMRR as used herein corresponds to the “optical-input-to-electrical-output” CMRR.
Painchaud et al (US 2011/0129213 A1) propose a method and apparatus for measurement of the overall CMRR for each differential output of the ICR. (It should be noted that Painchaud et al designate the term “Single Port Rejection Ratio”—SPRR—to describe the CMRR encompassing contributions of both the optical paths and the detectors responsivities.) FIG. 1 shows a schematic of this prior-art test setup suitable for such measurement on an ICR 10. A tunable laser 12 serves as a source of highly-coherent light at optical frequency ν0, and this light is then modulated in amplitude, by means of an intensity modulator (IM) 14 to generate two optical sidebands. These sidebands straddle the laser optical frequency ν0, respectively shifted to the “blue” and “red” sides therefrom by an optical frequency difference of fIF. The frequency fIF is precisely selected and known from the modulation frequency generated by an rf frequency generator 16, the electrical output of which is applied to the intensity modulator 14. The rf frequency generator 16 and the intensity modulator 14 generally must have a high electrical bandwidth, typically of the order of the symbol rate, fBAUD, at which the ICR-under-test is normally operated (e.g. approximately 30 GBaud for currently deployed PM-QPSK transmission systems), since CMRR characterization and determination of other parameters (e.g. Total Harmonic Distortion—THD) normally requires that measurements be carried out at rf frequencies spanning the ICR electrical bandwidth. In addition to the generally expensive hardware required for generating sidebands at fIF, the system of Painchaud et al (loc cit) also requires a slowly-varying controllable phase modulator 18 disposed in the optical path before one of the ICR inputs. Moreover, their method requires that the optical powers of the portions of test light launched simultaneously into the LO and SIG input ports be in a predetermined mutual proportionality (e.g. in a 2:1 ratio for their embodiment illustrated in FIG. 1), requiring the use of accurately calibrated attenuators 20 or other power control means.
It would thus be desirable that there be an alternative method and system to characterize the CMRR of an integrated optical receiver, in particular an “optical-input-to-electrical-output” CMRR. It is desirable that this alternative method be compatible with simpler and less expensive hardware, and, like the aforementioned prior-art approach, be compatible with the characterization of other key ICR parameters.